1. Field of the Invention
The present invention relates to a magnetoresistive element utilizing a tunnel magnetoresistive effect, and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each of which incorporates the magnetoresistive element.
2. Description of the Related Art
Improvements in performance of thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments in read heads using TMR elements have been sought to respond to further improvements in areal recording density.
Typically, a TMR element incorporates: a tunnel barrier layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the tunnel barrier layer; a pinned layer disposed adjacent to the other of the surfaces of the tunnel barrier layer; and an antiferromagnetic layer disposed adjacent to one of two surfaces of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer through which electrons are capable of passing while maintaining spins thereof by means of the tunnel effect. The free layer is a ferromagnetic layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
In the TMR element, the direction of magnetization in the free layer changes in response to the signal magnetic field sent from a recording medium, and accordingly, the relative angle between the direction of magnetization in the free layer and the direction of magnetization in the pinned layer changes. If the relative angle changes, the rate of electrons passing through the tunnel barrier layer while maintaining the spins thereof changes, and as a result, the resistance of the TMR element changes. It is possible to read data stored on the recording medium by detecting the change in resistance of the TMR element.
In the TMR element, theoretically, the ratio of magnetoresistive change to the resistance (hereinafter referred to as MR ratio) is expressed by:2P1P2/(1−P1P2)where P1 and P2 represent the spin polarizations of the free layer and the pinned layer, respectively. Therefore, the higher the spin polarizations P1 and P2, the higher is the MR ratio.
In view of this, to achieve a higher MR ratio in the TMR element, it would be useful to employ a ferromagnetic material having a high spin polarization as the material of the free layer and the pinned layer. However, the following problem arises if the free layer is made of a ferromagnetic material having a high spin polarization. Typical ferromagnetic materials having a high spin polarization include an alloy containing Co and Fe, and an alloy containing Ni and Fe wherein the proportion of Fe is higher. If such a ferromagnetic material is used as the material of the free layer, the soft magnetic property of the free layer deteriorates while a high MR ratio can be obtained. To be specific, deterioration of soft magnetic property means an increase in coercive force. To use a TMR element for a read head, it is required that the TMR element should have a high MR ratio and a high magnetic field sensitivity (magnetoresistive change/external magnetic field change). If the soft magnetic property of the free layer deteriorates as mentioned above, the magnetic field sensitivity of the TMR element is lowered and output signals of the TMR element tend to become unstable.
To solve the above-mentioned problem, Japanese Patent No. 3050189 B2 discloses a technique in which the free layer is made up of two layers, one of which is a high polarization layer that is in contact with the tunnel barrier layer and the other of which is a soft magnetic layer that is disposed such that the high polarization layer is sandwiched between the soft magnetic layer and the tunnel barrier layer.
According to the technique disclosed in the above-mentioned Japanese Patent, however, increasing the MR ratio of the TMR element causes an increase in the coercive force of the free layer, and reducing the coercive force of the free layer causes a reduction in the MR ratio of the TMR element. It is thus difficult to achieve both of a higher MR ratio of the TMR element and a lower coercive force of the free layer.